1. Field of the Invention
This invention relates to a pivotable mirror device for tracking used, for example, in an optical disk apparatus to cause a light beam spot to follow tracks on an optical disk.
2. Related Background Art
FIG. 1 of the accompanying drawings is a perspective view of an optical disk apparatus according to the prior art. In FIG. 1, the reference numeral 200 designates a base, the reference numeral 201 denotes a carriage supported on the base 200 for movement in the radial direction of an optical disk, not shown, the reference numeral 202 designates an objective lens supported on the carriage 201 by a leaf spring 204 and pivotable in the focusing direction of the optical disk, not shown, by an objective lens driving device 203, the reference numeral 205 denotes a pivotable mirror device for tracking which displaces a light beam emitted from a light source in the tracking direction of the optical disk, not shown, the reference numeral 206 designates a fixed optical system including a light source, a detector, PBS, etc., and the reference numeral 207 denotes a spindle motor on which the optical disk, not shown, is placed.
In the above-described construction, during the actual recording or reproduction of information, a light beam emitted from the light source is applied to the optical disk being rotated by a predetermined number of rotations by the spindle motor 207 while being tracking-controlled and focusing-controlled by the use of the pivotable mirror device for tracking and a focusing drive device, whereby the recording or reproduction of the information is effected.
FIG. 2 of the accompanying drawings is a perspective view showing the details of the pivotable mirror device 205 for tracking in FIG. 1.
Referring to FIG. 2, a mirror 1 is pivotably supported by a thin leaf spring 3 through a mirror holding member 2, and is designed to be pivotablly driven up and down by a yoke 4, a permanent magnet, not shown, and a coil attached to the rear of the mirror holding member 2. In FIG. 2, the constricted portion of the thin leaf spring 3 functions as a torsional portion, but a similar function may be performed by a wire material.
In the above-described example of the prior art, however, there is very little attenuation element for torsional vibration and therefore, the attenuation ratio (C/Cc) at the resonance frequency is small and as in the portion indicated by broken line in FIG. 3 of the accompanying drawings, the resonance amplitude becomes great. Therefore, a filter circuit of high order such as a notch filter is required in the control circuit of the pivotable mirror device for tracking, and this leads to the problems that the control circuit becomes complicated and that an apparatus carrying such device thereon requires a vibration control mechanism of extremely low resonance frequency and it is difficult to make the apparatus compact.